[release-branch.go1.9] cmd/compile: fix constant folding of right shifts
    
    The sub-word shifts need to sign-extend before shifting, to avoid
    bringing in data from higher in the argument.
    
    Fixes #23812
    
    Change-Id: I0a95a0b49c48f3b40b85765bb4a9bb492be0cd73
    Reviewed-on: https://go-review.googlesource.com/93716
    Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
    TryBot-Result: Gobot Gobot <gobot@golang.org>
    Reviewed-by: Cherry Zhang <cherryyz@google.com>
    Reviewed-on: https://go-review.googlesource.com/102775
    Run-TryBot: Andrew Bonventre <andybons@golang.org>